Medication delivery systems are known in the art. Medication delivery systems are used to deliver pain control medication and other medications intra-operatively, subcutaneously, and percutaneously to a patient after a surgical, or some other medical, procedure.
It is sometimes desirable to deliver a fluid using a pulsatile fluid flow or series of pulses. For example, some medication delivery systems which utilize a series of pulsatile fluid pulses to deliver medication, are known in the art. Medication delivery systems may be used to deliver pain control medication and other medications intra-operatively or post-operatively, subcutaneously, and percutaneously to a patient after a surgical, or some other medical, procedure.
For example, U.S. Pat. No. 5,807,075 to Jacobsen et al. discloses a conventional medication delivery system that includes a base housing and a cassette. The base housing of the '075 patent houses electronic components, such as an electric motor, a power source, and an electronic controller, and the cassette of the '075 patent interacts with a supply of the medication to deliver the medication to the patient.
A further example of a conventional medication delivery system is disclosed in U.S. Pat. No. 4,650,469 to Berg et al. This patent discloses a medication delivery system that includes a control module and a reservoir module removably connected to the control module. The control module includes a pump mechanism, valves, a power source, electronic controls, and the like, and the reservoir module includes a container that supplies the medication to be delivered to the patient.
It is known to use an electric motor in such medication delivery systems, where a predetermined number at revolutions or cycles of the motor delivers a preset amount of medication. Such systems are known as positive displacement systems. In such systems, pressurization of the medication is a function of the restrictions in the flow path and the time dependent flow of medication through the system.
Generally, conventional medication delivery systems provide a flow of medication through an output tube which then is delivered to the patient, as required. However in some procedures, medication is required at two locations with respect to the patient, for example, breast augmentation or reconstruction. Another such procedure where medication delivery is desirable at two sites is an autologous graft procedure where it is desirable to deliver medication at both the graft and the donor sites. If the medication provided by the delivery system is pumped through a “Y” connection, then there are several reasons that the medication may not be delivered to each site or location in the desired proportion. First, unequal pressure at the two infusion sites due to elevation or intracompartmental pressure sets up a siphon where flow occurs from one side to the other side in the period between pulses. Furthermore, natural or unintended variations in flow restriction between the two sides of the “Y” and/or the previously mentioned unequal infusion site pressures may shift the proportion of the flow split, as more flow will follow the path of decreased resistance. This is undesirable.
In a mechanical system experiencing laminar flow of a non-compressible fluid, a similar phenomenon occurs whereby the pressure between two points is directly proportional to the mass flow rate through the system and the flow restriction between the two points.
This can be expressed as ΔP={dot over (M)}R, where
ΔP=Pressure Differential (psi)
{dot over (M)}=Mass Flow Rate (cc/sec)
R=Flow Restriction (psi/[cc/sec])
Similarly,
      M    .    =            Δ      ⁢                          ⁢      P        R  
In other words, the instantaneous flow rate in a single-lumen system is directly related to the instantaneous pressure between two points separated by a known flow restriction, and it is inversely proportional to the value of the restriction between those two points.
In a scenario where more than one distal site is linked to the fluid path, the overall flow rate to both sites as well as the percent flow reaching each site is also related to pressure and restriction. Though capacitance in the system may cause a phase shift in the instantaneous flow rate from location to location, the overall flow from the lumen upstream of the branching node will equal the sum of the overall flow coming through each outlet lumen (leg). z
In this scenario, the instantaneous flow rate along each leg will be directly related to the difference in pressure between the branching node and the distal outlet of the leg, and it will be inversely proportional to the flow restriction along that leg. The mass flow rate through any given outlet lumen may be calculated as follows:
            M      .        i    =                    Δ        ⁢                                  ⁢                  P          i                            R        i              =                            P          0                -                  P          i                            R        i            whereΔPi=P0−Pi
P0=Pressure at branching node (psi gage)
Pi=Pressure at outlet of lumen i (psi gage)
Ri=Flow Restriction of outlet lumen i (psi/[cc/sec])
The total flow through all legs is then
                    M        .            total        =                            ∑                      i            =            1                    n                ⁢                              M            .                    i                    =                        ∑                      i            =            1                    n                ⁢                                            P              0                        -                          P              i                                            R            i                                ,and the percent flow to any given outlet lumen is
      %    ⁢                  ⁢    F    =            (      100      )        ⁢                                        M            .                    i                                      M            .                    total                    .      
If a pump with a pulsatile flow delivery system is connected to a fluid delivery path, and it is desired to controllably divide the flow between the multiple outlet sites, the flow restrictions in each outlet lumen may be designed so as to facilitate the desired flow distribution. With each pulse of fluid flow from the pump, the pressure in the inlet lumen and the branching node will rise—with a lower system capacitance before the node leading to a more steep pressure rise. After the pump finishes introducing fluid to the path, a pressure drop will be observed as fluid drains through the outlet lumens, emptying the fluid stored by the capacitance of the system. As stated before, the instantaneous flow rate during this process is related to the pressure differential and the flow restriction; as the pressure drops asymptotically to an equilibrium level, so will the flow decrease proportionally.
If the pressure at all outlet sites is the same, then ΔP will be the same for each outlet lumen and the flow distribution may be directly controlled by the flow restrictors alone.
However, if the pressures of the outlets are not equal and not constant, then additional measures are required to equally or accurately distribute the flow as desired to all sites. This can be accomplished by making the pressure at the node high enough that the variation in pressures at each site do not contribute greatly to percent variation in pressure differential. In other words, if P1≠P2 but Po>>P1 and Po>>P2, then ΔP1≈ΔP2. If ΔP1 and ΔP2 are then similar, the restriction level of each lumen may once again be relied upon to provide the control necessary to balance the flow percentage to each lumen outlet.
However, in the case of a pulsatile pump, the time-dependent pressure profile may not provide the necessary conditions to keep Po>>P1 and Po>>P2. A substantial portion of fluid flow occurs as the pressure profile drops during the drain cycle mentioned above, and as the pressure drops, the ΔPi of the various paths will deviate farther and farther from each other, in relation to the difference in Pi at each outlet location.
One solution would be to provide a check valve in each leg after the “Y” connection. This solution presents several problems, namely, there is a time delay added by the opening and closing of the check valve and differences in manufacturing tolerances contributing to the delay may also lead to uneven delivery of the medication. Furthermore, most check valves restrict flow when open, and unequal or uncontrollable variations in this restriction would lead to unequal flow.
Another solution would be to provide a large fluid resistor (small orifice) in each leg. Correctly sizing this orifice would cause the pressure to rise substantially higher than the downstream pressure differences. This pressure could be driven up over several pulses. If the pressure remained higher than the highest downstream pressure, no backflow due to siphoning could occur. Furthermore, the difference in the pressure drop in the two downstream legs could be controlled to remain relatively equal. This solution presents several problems. First, if the pump has a user selectable flow rate, the size of the glass orifice must be fixed to work with the lowest possible flow rate. If a higher flow rate were then selected, the level of restriction would cause an increase in pressure beyond acceptable limits for safety or function the system or its components, including features such as an occlusion sensor.
The present invention is aimed at one or more of the problems set forth above.